1. Field of the Invention
The present invention relates to an apparatus and a method for coating a coating solution on for example a substrate and forming a soft silicon film.
2. Description of the Related Art
As a method for forming an inter-layer insulation film of a semiconductor device, CVD method and heat oxidizing method are known. In addition, sol-gel method is known. In the sol-gel method, coating solution of which colloid of TEOS (tetraethoxysilane: Si(C.sub.2 H.sub.5 O).sub.4) has been dispersed in organic solvent such as ethanol solvent is coated on the front surface of a semiconductor wafer (hereinafter simply referred to as wafer). The coated film is gelated and dried. Thus, a silicon oxide film is obtained. This method has been disclosed in for example Japanese Patent Laid-Open Publication Nos. 8-162450 and 8-59362.
FIGS. 19A to 19C show states of denaturation of the coated film in the sol-gel method. When the coating solution is coated on the wafer, particles or colloid 100 of TEOS is dispersed in solvent 200 (see FIG. 19A). Thereafter, the coated film is exposed to alkali atmosphere. Thus, since TEOS is poly-condensed and hydrolyzed, the coated film is gelated and thereby a mesh structure of TEOS 300 is formed (see FIG. 19B). Thereafter, solvent contained in the coated film is substituted with another solvent 400 so as to remove moisture of the coating solution (see FIG. 19C). The coated film is dried. Thus, a soft silicon film is obtained as the coated film. In the solvent substituting process shown in FIG. 19C, in addition to removing moisture, with solvent whose surface tension is smaller than that of ethanol, large force is prevented from being applied to the mesh structure of TEOS. Thus, the structure of the film can be prevented from breaking.
When such sol-gel method is applied for a real fabrication line, a coating unit that coats a coating solution to a wafer, a gelating unit that gelates the coated film, and a solvent substituting unit that substitutes solvent contained in the coated film with another solvent are required. In addition, a pre-process unit that performs a preprocess such as a hydrophobic process for a wafer and a baking unit that dries the wafer are required. Moreover, a conveying mechanism that conveys a wafer among these units is required.
After coating solution is coated on a wafer, solvent should be suppressed from evaporating so as to prevent the thickness of the resultant film from decreasing. In addition, after TEOS is gelated until the solvent substituting step is preformed, the solvent should be suppressed from evaporating so as to prevent strong force from being applied to the mesh structure of TEOS.
However, the process time period varies unit by unit. In addition, the process time period also varies corresponding to the unit structure. In other words, in the case that many wafers conveyed with a cassette are successively processed, when a wafer that has been processed in a particular unit is converted to another unit, a waiting time period is required. F or example, a wafer that has been processed in the coating unit may not be converted to the gelating unit, but stayed in the coating unit. Alternatively, a wafer that has been processed in the gelating unit may not be converted to the solvent substituting unit, but stayed in the gelating unit. When such a waiting time period takes place, the solvent on the wafer evaporates. Thus, the film quality deteriorates.
In addition, when a wafer is conveyed from one unit to another unit, the wafer is exposed to air. Thus, when a wafer that has been processed in the coating unit is conveyed to the gelating unit, the solvent on the front surface of the wafer evaporates and thereby the film quality deteriorates.